The purpose of a catalytic converter for an internal combustion engine or a gas turbine is to convert pollutant materials present in the exhaust, e.g., carbon monoxide, unburned hydrocarbons, nitrogen oxides, etc., to carbon dioxide, nitrogen and water. Conventional automotive catalytic converters utilize an oval or round cross-section ceramic honeycomb monolith having square or triangular straight-through openings or cells with catalyst deposited on the walls of the cells; catalyst coated refractory metal oxide beads or pellets, e.g., alumina beads; or a corrugated thin metal foil multicelled honeycomb monolith, e.g., a ferritic stainless steel foil honeycomb monolith, or a nickel alloy foil honeycomb monolith, having a refractory metal oxide coating and catalyst carried thereon or supported on the surface of the cells. The catalyst is normally a noble metal, e.g., platinum, palladium, rhodium, ruthenium, or a mixture of two or more of such metals. Zeolite coatings may also be used for the absorption and desorption of the pollutants to aid in catalytic activity. The catalyst catalyzes a chemical reaction, mainly oxidation, whereby the pollutant is converted to a harmless by-product which then passes through the exhaust system to the atmosphere.
However, conversion to such harmless by-products is not efficient initially when the exhaust gases are relatively cold, e.g., at cold engine start. To be effective at a high conversion rate, the catalyst and the surface of the converter which the exhaust gases contact must be at or above a minimum temperature, e.g., 390 F. for carbon monoxide, 570 F. for volatile organic compounds (VOC), and about 900 F. for methane or natural gas. Otherwise conversion to harmless by-products is poor and cold start pollution of the atmosphere is high. It has been estimated that as much as 80% of the atmospheric pollution caused by vehicles, even though equipped with conventional non-electrically heated catalytic converters, occurs in the first 2 minutes of operation of the engine. Once the exhaust system has reached its normal operating temperature, an unheated catalytic converter is optimally effective. Hence, it is necessary for the relatively cold exhaust gases to make contact with hot catalyst so as to effect satisfactory conversion. Compression ignited engines, spark ignited engines, reactors in gas turbines, small bore engines such as used in lawn mowers, trimmers, boat engines, and the like, have this need.
To achieve initial heating of the catalyst at engine start-up, there is conveniently provided an electrically heatable catalytic converter unit, preferably one formed of a thin metal honeycomb monolith. This monolith may be formed of spaced flat thin metal strips, straight corrugated thin metal strips, pattern corrugated thin metal strips, (e.g., herringbone or chevron corrugated), or variable pitch corrugated thin metal strips (such as disclosed in U.S. Pat. No. 4,810,588 dated 7 Mar. 1989 to Bullock et al) or a combination thereof, which monolith is connected to a voltage source, e.g., a 12 volt to 108 volt or higher, AC or DC power supply, preferably at the time of engine start-up and afterwards to elevate the catalyst to and maintain the catalyst at at least 650 F. plus or minus 30 F. Alternatively, power may be also supplied for a few seconds prior to engine start-up.
Catalytic converters containing a corrugated thin metal (stainless steel) monolith have been known since at least the early 1970's. See Kitzner U.S. Pat. Nos. 3,768,982 and 3,770,389 each dated 30 Oct. 1973. More recently, corrugated thin metal monoliths have been disclosed in U.S. Pat. No. 4,711,009 dated 8 Dec. 1987; U.S. Pat. No. 4,381,590 dated 3 May 1983 to Nonnenmann et al, U.S. Pat. No. 5,070,694 dated 10 Dec. 1991 to Whittenberger; and International PCT Publication Numbers WO 89/10470 (EP 412,086) and WO 89/10471 (EP 412,103) each filed 2 Nov. 1989, claiming a priority date of 25 Apr. 1988. The above International Publication Numbers disclose methods and apparatus for increasing the internal resistance of the device by placing spaced discs in series, or electrically insulating intermediate layers. Another International PCT Publication Number is WO 90/12951 published 9 Apr. 1990 and claiming a priority date of 21 Apr. 1989, which seeks to improve axial strength by form locking layers of insulated plates. Another reference which seeks to improve axial strength is U.S. Pat. No. 5,055,275 dated 8 Oct. 1991 to Kannainian et al. Reference may also be had to PCT Publication Number WO 92/13636 claiming a priority date of 31 Jan. 1991. This application relates to a honeycomb body along an axis of which a fluid can flow through a plurality of channels. The honeycomb body has at least two discs in spaced relation to each other. According to the specification, there is at least one support near the axis by which the discs are connected together and mutually supported. The invention is said to make possible design of the first disc for fast heating up through exhaust gas passing through or applied electrical current. The honeycomb body serves as a bearer for catalyst in the exhaust system of an internal combustion engine. Another reference is German Patent Application number 4,102,890 A1 filed 31 Jan. 1991 and published 6 Aug. 1992. This application discloses a spirally wound corrugated and flat strip combination wherein the flat strip contains slots and perforations and is electrically heatable. The flat strips include a bridge between leading and trailing portions. Groups of such strips are separated by insulation means. The core is provided with a pair of circular retainer segments which are separated by insulation means. No end tabs are provided, and the flat strip portions are integral. A principal difference between the German Application and the present case is that the electrical current flow through the heater in the reference is "nonhomogeneous" whereas in the present case the electrical current flow is homogeneous.
A common problem with such prior devices has been their inability to survive severe automotive industry durability, or proof, tests which are known as the Hot Shake Test and the Hot Cycling Test.
The Hot Shake Test involves oscillating (100 to 200 Hertz and 28 to 60 G inertial loading) the device in a vertical attitude at high temperature (between 800 and 950 C.; 1472 to 1742 F., respectively) with exhaust gas from a running internal combustion engine simultaneously passing through the device. If the catalytic device telescopes or displays separation or folding over of the leading or upstream edges of the foil leaves up to a predetermined time, e.g., 5 to 200 hours, the device is said to fail the test. Usually a device that lasts 5 hours will last 200 hours. Five hours is equivalent to 1.8 million cycles at 100 Hertz.
The Hot Cycling Test is conducted with exhaust flowing at 800 to 950 C. (1472 to 1742 F.) and cycled to 120 to 150 C. once every 15 to 20 minutes, for 300 hours. Telescoping or separation of the leading edges of the thin metal foil strips is considered a failure.
The Hot Shake Test and the Hot Cycling Test are hereinafter called "Hot Tests," and have proved very difficult to survive. Many efforts to provide a successful device have been either too costly or ineffective for a variety of reasons.
The structures of the present invention will survive these Hot Tests.
Early embodiments of electrically heatable catalytic converters were relatively large, especially in an axial direction, e.g. 7 to 10 or more inches long and up to 4.5 inches in diameter. These were inserted into an exhaust system either as a replacement for the conventional catalytic converter now in common use, or in tandem relation with such conventional catalytic converter in the exhaust line. Then it was found that an axially relatively thin or "pancake," electrically heated corrugated thin metal honeycomb monolith could be used in close tandem relation with the conventional catalytic converter.
It was later found that even better performance resulted from a "cascade" of converters, i.e., a low thermal inertia electrically heatable converter (EHC), followed by a medium thermal inertia converter, followed by a large thermal inertia main converter, all on generally the same axis of gas flow. This solution provided for fast, economical heating of the EHC. Heat generated from an oxidation reaction initiated in the EHC then heated the intermediate converter which in turn heated the large converter.
It should be noted that the electrically heatable honeycomb acts to preheat the exhaust gas to its "light-off" temperature where, in the presence of catalyst, the pollutants are converted. Some conversion occurs in the electrically heatable device, and most of the conversion occurs in the final catalytic converter section which is normally not electrically heated.
It has now been found that a "pancake" electrically heatable device and a conventional metal monolith catalytic converter may be positioned together within a common housing to take advantage of the common diameter and/or geometric configuration in a cascading device, and having a shorter axial length than required in either the tandem relation or the prior cascade relation. While in the present devices there are still three units of differing thermal inertia in a "cascade," instead of a three structural member cascade device, the present structure enables a two member cascade device. A still further advantage of the present device is that it facilitates manufacture from thin metal strips, to form both the electrically heatable portion and the conventional metal monolith catalytic converter portion, or "light-off" portion, for encasing in a single housing. The dual purpose devices of the present invention may be backed up with a conventional catalytic converter of, for example, the commonly used ceramic type, the alumina pellet type, or the metal monolith honeycomb type mentioned above. Thus, the advantages of the cascade effect for successive light-off may be utilized without encountering a number of the problems associated therewith. Avoiding substantial electrical heating of the major portion of the thin metal honeycomb effects a major saving in electrical power required, which is one reason for the "pancake," or axially relatively thin, e.g., less than 2", structure in the first place. The small thermal mass of the "pancake" device enables very short heat up times, i.e., a matter of a very few seconds. Oxidation of the pollutant materials in the exhaust gas is initiated in the "pancake" portion and the resulting exotherm further heats the exhaust gas and the subsequent "light-off" converter to effect substantial completion of the oxidation of the pollutant materials in the presence of a catalytic agent or agents.
In the following description, reference will be made to "ferritic" stainless steel. A suitable ferritic stainless steel alloy is described in U.S. Pat. No. 4,414,023 dated 8 Nov. 1983 to Aggen. A specific ferritic stainless steel useful herein contains 20% chromium, 5% aluminum, and from 0.002% to 0.05% of at least one rare earth metal selected from cerium, lanthanum, neodymium, yttrium, and praseodymium, or a mixture of two or more of such rare earth metals, balance iron and trace steel making impurities. Another metal alloy especially useful herein is identified as Haynes 214 alloy which is commercially available. This and other nickeliferous alloys are described in U.S. Pat. No. 4,671,931 dated 9 Jun. 1987 to Herchenroeder et al. These alloys are characterized by high resistance to oxidation. A specific example contains 75% nickel, 16% chromium, 4.5% aluminum, 3% iron, optionally trace amounts of one or more rare earth metals except yttrium, 0.05% carbon, and steel making impurities. Haynes 230 alloy, also useful herein, has a composition containing 22% chromium, 14% tungsten, 2% molybdenum, 0.10% carbon, and a trace amount of lanthanum, and balance nickel. Ferritic stainless steel (commercially available from Allegheny Ludlum Steel Co. under the trademark "Alfa IV") and the Haynes alloys are examples of high temperature resistive, oxidation resistant (or corrosion resistant) metals that are suitable for use in making thin metal strips for use in the converters hereof, and particularly for making heater strips for cores that may be electrically heated. Suitable metals must be able to withstand temperatures of 900 C. to 1100 C. over prolonged periods.
Other high temperature resistive, oxidation resistant resistant metals are known and may be used herein. For most applications, and particularly automotive applications, these alloys are used as "thin" metal strips, that is, having a thickness of from 0.001" to 0.005", and preferably 0.0015" to 0.0025".
In the following description, reference will also be made to fibrous ceramic mat, woven ceramic fiber tape or fabrics, or insulation. Reference may had to U.S. Pat. No. 3,795,524 dated 5 Mar. 1974 to Sowman and to U.S. Pat. No. 3,916,057 dated 28 Oct. 1975 to Hatch, for formulations and manufacture of ceramic fiber tapes and mats useful herein. One such woven ceramic fiber material is currently available from 3-M under the registered trademark "NEXTEL" 312 Woven Tape and is useful for insulation of thin metal strips as described below. Ceramic fiber mat is commercially available as "INTERAM" also from 3-M.